This invention relates to leuco dyes, and more particularly, to stabilized leuco phenazine dyes.
It is well known that dyes in their reduced leuco form can provide the basis of color image forming systems. The leuco dyes may initially be relatively colorless, but can return to a colored form when oxidized, e.g., by nitrate salts in the presence of heat. Examples of leuco dyes used in color image forming systems include triarylmethanes, xanthenes, styryl dyes, and azine dyes, such as, for example, phenazines, phenoxazines, and phenothiazines.
It is also known that the leuco form of a given dye may suffer from instability under ambient conditions and can often revert spontaneously to the colored form of the dye. Among the best of the leuco dyes with respect to thermal and photochemical stability are the phenoxazine and phenothiazine dyes. However, the phenoxazine and phenothiazine dyes are normally restricted to blue and turquoise colors. The phenazine leuco dyes are considerably less stable than the phenoxazine and phenothiazine dyes. However, phenazine dyes are capable of providing yellow, orange, red, and magenta colors. It would be extremely desirable to obtain stable leuco derivatives of the phenazine class in order to provide yellow, orange, red, magenta, and purple dye forms.
In previous thermographic systems based on combinations of leuco dyes and nitrate salts, at least two approaches have been applied to address the problem of stability to both thermal and actinic effects. The thermal effect, with respect to thermographic processes for preparing transparencies for overhead projection, refers to the problem of attaining the highest possible thermal speed difference between temperatures ranging from 100.degree. C. to 200.degree. C., which range is typical of the temperatures required for image exposure, and temperatures ranging from 25.degree. C. to 60.degree. C., which range is typical of the temperatures on the projector stage after prolonged operation. The actinic effect refers to problems associated with the high light intensity to which the finished transparency is subjected on the projector stage.
One approach involves adding to the imaging system stabilizing compounds, which, in most cases, are normally mild antioxidants, such as, for example, phenidone or ascorbic acid. A second approach involves altering the structure of the leuco dye. Although the first approach has sometimes been successful, it often leads to loss of sensitivity at the temperatures required for image exposure. In many cases, it is not effective at all. Therefore, the second approach is preferable.
Examination of the prior art indicates that acylated (benzoylated) leuco azine dyes exhibit improved stability compared with the hydrogen leuco form thereof. Benzoylated leuco forms of phenoxazines and phenothiazines often show extremely good stability; however, benzoylated leuco phenazine dyes are still far too unstable to be useful for most types of thermally imageable compositions.